All radio frequency transmission and/or receiver devices require an antenna to accomplish communication connection. While conventional whip antennas commonly found on vehicles for standard radio reception or for receipt and transmittal of information are commonly used and satisfactory for such use, certain applications do not lend themselves to this type of antenna construction.
Specifically, where space consideration is critical or where visible antenna presence is not desired, a whip antenna cannot be accommodated. When such is the case, flat or patch antennas are employed.
Flat antennas are generally constructed of two generally parallel conductive sheets with the top sheet being the resonant element and the bottom sheet being the ground plane. An antenna can be either linearly or circularly polarized depending upon resonant dimensioning and feed-point characteristics. Depending upon the geographic relationship between an antenna and a receiver, linearly polarized transmissions may go unreceived if nonalignment or cross polarization occurs between the antenna and receiver. A circularly polarized antenna, however, generally reduces polarization mismatch to thereby maintain link integrity between the antenna and the receiver. In either polarity, however, when an antenna constructed according to the prior art comes in close proximity to a metal object, such as a metal wall for example, antenna impedance is adversely affected which, of course, adversely affects system performance. In like manner, impedance and resultant transmission/reception of loop or wire antennas employed in, for example, personal monitoring tag devices, is unfavorably affected in the presence of metal.
In view of the above transmission and reception difficulties encountered in the presence of a metal object, and in view of a requirement that a monitor tag must be operable both in and out of the presence of metal, it is apparent that a need is present for a monitor tag with a patch antenna having the capability to transmit and/or receive radio signals without interference from environmental surroundings that include metal objects. Accordingly, a primary object of the present invention is to provide a monitor tag having a patch antenna whose transmission and reception capabilities are substantially unaffected by the proximity of metal objects.
Another object of the present invention is to provide a monitor tag having a patch antenna wherein in the antenna generally parallel conductive sheets are separated by a dielectric material having a thickness sufficient to create a gap between the patches where the radiation resistance will be controlled during operation of the antenna.
Yet another object of the present invention is to provide a monitor tag with a patch antenna whose resonant dimension is chosen from one-quarter and one-half wavelength and with appropriate feed-point locations to thereby provide either linear or circular polarity.
Still another object of the present invention is to provide a monitor tag with a patch antenna having a back lobe radiation pattern to aid in dispersion or reception of a signal.
These and other objects of the invention will become apparent throughout the description of the invention which now follows.